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An analysis of the work processes of manipulators of technological machines has shown insufficient reliability of metal structures and hydraulic drive elements. One of the ways to increase the reliability of manipulator-type forest machines is to reduce dynamic loads through the use of various damping devices. The aim of the study is to reduce the dynamic load and increase the reliability of the boom lifting mechanism of the hydraulic manipulator of the timber transport machine. The authors propose a new boom lifting mechanism with a pneumohydraulic damping device, including a pneumohydroaccumulator and a plunger-type damper, protected by patents for invention and utility model. A mathematical model of the lifting mechanism of a forest manipulator with a pneumohydraulic damping device has been developed, which includes three equations: the equation of rotational motion of the boom, the equation of the flow rate of the working fluid, including the flow rate of the pneumatic accumulator, and the equation of motion of the damper plunger. The solution to the problem is found by decomposing the function into a Maclaurin series. Using the final formulas, pressure graphs of the working fluid were obtained and the dynamic coefficients were calculated without connecting the pneumatic accumulator to 1.65, and with 1.35 connection, i.e. a decrease in dynamic load by 18.5%. Experimental studies were carried out on a laboratory stand with a serial LV-210-01 forest hydraulic manipulator using the method of a full-factor experiment. As a result of processing the obtained waveforms, statistical characteristics of the pressure of the working fluid in the piston cavity of the boom drive hydraulic cylinder for various boom spans and damper choke diameters were obtained without inflating the pneumatic accumulator and with connection. It was found that without the connection of a pneumatic accumulator, the average coefficient of dynamism is 1.55, and with the connection, the average coefficient of dynamism is 1.32, i.e., according to experimental data, a decrease occurs by 23%, a discrepancy with theoretical by 4.5%. The calculation of the dependence of the probabilistic safety coefficient on the operating time of the LV-210-01 forest hydraulic manipulator showed that without connecting a pneumatic hydraulic accumulator, the permissible value of the safety coefficient (1.5-2.0) is reached in 15 years, while with the connection of a pneumatic hydraulic accumulator in a damping device, it allows extending the service life over 20 years. The results obtained are of practical importance for assessing the dynamic load and reliability of manipulators of forest machines at the design stage and can be used in design organizations of the forestry engineering.